The present invention relates to the art of open circuit emergency breathing apparatus of the character including a pressure demand valve and, more particularly, to an improved pressure demand valve and control arrangements for supplying breathing gas from a source to a user.
The present invention finds particular utility in connection with open circuit emergency breathing apparatus of the character which is known as a supplied air system with an escape feature. More particularly, such apparatus is intended to be connected to a primary supply of breathing gas under pressure in a location where the apparatus is to be used, such breathing gas supply being from an air compressor for example. Generally, a supply hose of considerable length, such as 50 feet for example, connects the primary supply with the breathing apparatus to enable the user to work in a large area in the locale. The emergency breathing apparatus is also adapted to be connected to a secondary source of breathing gas under pressure in a container carried or worn by the user. The second source enables disconnection of the apparatus from the primary source and provides the user with a sufficient supply of breathing gas to enable the user to escape from or evacuate the area in which he or she was working. Such a secondary supply container generally provides a 5 to 10 minute supply of breathing gas for such purpose. In either instance, the breathing gas is supplied to a user, such as by a face mask worn by the user, and the breathing gas inhaled by the user is exhaled to atmosphere from the face mask. Further, the breathing gas is supplied to the face mask through a pressure demand valve which is responsive to the user's rate of inhaling and exhaling, or breathing rate, so as to provide the appropriate flow of breathing gas to the user to meet the latter's needs as determined by such breathing rate.
As is well known, the breathing rate of the user of such emergency breathing apparatus, measured for example in breaths per minute, can vary considerably during use in response for example to increases and decreases in the user's physical exertion. The pressure demand valve, therefore, must be capable of supplying the appropriate volume of breathing gas upon each inhalation of the user regardless of the time of and the time between succeeding inhalations. The volume of breathing gas required by the user, known as the tidal volume, is that volume of breathing gas required to fill the user's lungs when he or she inhales and, accordingly, is dependent on lung capacity and the extent to which the user inhales relative to his or her lung capacity. At low breathing rates, of about 15 or 20 breaths per minute for example, such as when the user is not undergoing significant physical exertion, the time required for one inhaling and exhaling cycle is relatively long and the volume of air inhaled per breathing cycle is relatively higher in comparison with the cycle time and volume per cycle inhaled when the user is undergoing some physical exertion and is breathing at a medium rate of, for example, 20 to 30 breaths per minute. In the latter situation, the time of each breathing cycle and the volume of breathing gas inhaled each breathing cycle decreases. When the user is undergoing considerable physical exertion and is breathing at a high rate of, for example, 30 to 45 breaths per minute the breathing cycle time further decreases as does the volume of breathing gas inhaled each breathing cycle. Accordingly, in connection with emergency breathing apparatus, it will be appreciated that the pressure demand valve must respond to such variations in the breathing rate and volume inhaled per breathing cycle to assure that the user of the apparatus has the required supply of breathing gas at the face mask, to assure that the supply is at a uniform flow rate at any given breathing rate, and to assure that the pressure in the face mask is maintained above atmospheric pressure so that the inhaling portion of the user's breathing cycle does not draw ambient air into the face mask. Furthermore, in connection with supplied air escape type breathing apparatus, the pressure demand valve must be capable of performing in the foregoing manner over a wide range of breathing gas supply pressures. In this respect, breathing gas supplied by the primary source is at a fixed pressure, but from one location to another the pressure may vary from 35 psi to 135 psi, for example. The pressure of breathing gas in the secondary supply container providing the escape source is fixed through a pressure reducing valve associated with the supply container and, generally, is between 120 psi and 135 psi.
Pressure demand valves for use with open circuit type emergency breathing apparatus generally include a diaphragm actuated flow control valve which opens and closes in response to inhaling and exhaling by the user such that breathing gas from the source flows to the face mask worn by the user during inhaling and stops during exhaling. Such pressure demand valves heretofore provided for use with open circuit type emergency breathing apparatus have most often been mounted on the face mask, and there are a number of disadvantages of such a mounting arrangement which either limit acceptability of the equipment and/or endanger the safety of the user and impair the efficiency of the latter in connection with his or her efforts to perform work while wearing the face mask. More particularly in this respect, mounting of a pressure demand valve on the face mask requires the use of plastic materials in connection with the construction of the valve in order to minimize the discomfort to the wearer of the mask as a result of the weight of the valve. Plastic materials undesirably subject the valve to damage and possible inoperativeness by impacting of the valve against objects during use and/or storage of the face mask. Further, the location of the valve on the face mask undesirably limits and/or obstructs visibility of the user, especially in connection with the user's looking downward while wearing the mask
In addition to the foregoing disadvantages, pressure demand valves heretofore provided for use with supplied air escape apparatus have not been effective to provide a uniform rate of breathing gas flow to the user's face mask in connection with the variables referred to above with respect to the user's breathing rate, tidal volume requirements, and the pressure of the breathing gas supply. More particularly in this respect, the pressure demand valve has a valve controlled orifice of given size which determines the volume of breathing gas which can flow through the valve to the face mask, and the pressure demand valves heretofore provided have been mounted on the face mask at least in part for the purpose of providing the response time necessary to assure the appropriate supply of breathing gas therethrough to the user in response to varying breathing rates and tidal volumes. Such previous pressure demand valves operate with acceptable effectiveness when the breathing gas supply pressure is relatively high and, for example, above 60 psi. At lower supply pressures, however, the pressure demand valves cannot operate effectively unless the valve actuating diaphragm and flow orifice are enlarged to enable the necessary volume of flow thereacross to the user. It will be appreciated that to enlarge the diaphragm of a face mask mounted pressure demand valve would further add to the weight and visibility problems mentioned above.